forward signals

Follow up from #1350: we now create a fake process for each server pid and we will receive signals sent by pagers and window managers, we should forward those to the real server-side process and let it decide what to do with it.

This will require:

a new capability: "window.signals"

a new server message: ["window-signal", wid, signal] (we send the wid and let the server find the real pid)